The liquid distribution and the kinetics of liquid redistribution is vital information that can be used to compare the functionality of different absorbent articles, such as hygiene products, and to develop new products. Currently, several techniques such as X-Ray and MRI are utilized to characterize the 3D liquid distribution within hygiene products. However, there is currently no known process to provide a fast data acquisition process in combination with a high resolution process at low cost to follow the kinetics of fluid movement inside absorbent articles.
NMR is a physical phenomenon based on the principle of exciting nuclear spins with radiofrequency pulses, the frequency of which matches the Larmor frequency of the nuclear spins. In other words, NMR is based on the nuclear magnetic properties of certain elements and isotopes of those elements. It is based on the principle that nuclei with a non-zero spin will have a magnetic dipole and therefore will interact with electromagnetic (EM) radiation.
This principle has been applied in many different research fields such as chemical structure analysis, materials testing and in medicine. To access certain nuclear spins via radiofrequency pulses, the nuclei have to be exposed to a magnetic field. These magnetic fields can be classified as high and low. Low magnetic fields as created by permanent magnets can yield magnetic field strengths up to 85 MHz proton Larmor frequency, whereas superconducting high field magnets reach drastically higher field strengths. Magnetic fields can either be produced by electrical currents running through cryogenically cooled coils of superconducting wires without resistance or by the use of permanent magnets.
The presence or absence of a spin and the nature of this spin is expressed in terms of the spin quantum number of the nucleus, which may either be 0, ½ or multiples of ½. In a uniform magnetic field a nucleus having a spin quantum number of ½ may assume two orientations relative to the applied magnetic field. The two orientations have different energies so that it is possible to induce a nuclear transition by the application of electromagnetic radiation of the appropriate frequency. This transition is resonance. Resonance arises when the correct combination of magnetic field strength and exciting frequency characteristics of the nuclei of interest are applied.
After resonance is achieved the NMR instrument records a signal, the signal being a function of the nature and amount of excited nuclei within the test sample as well as nuclear magnetic relaxation considerations. A NMR device generally comprises one or more magnets producing a strong homogenous field in combination with gradients within a test region to be applied for imaging, spectroscopy or relaxometry. The size and complexity of NMR spectrometers are largely a function of the magnetic field requirements. In contrast to NMR applications requiring homogenous magnetic fields, single sided, or open, NMR devices make use of inhomogeneous magnetic fields having highly uniform gradients.
One particular early form of single sided NMR, the NMR-Mobile Universal Surface Explorer (NMR-MOUSE) was introduced in 1995. The early design of the NMR-MOUSE was limited to a maximum penetration depth of less than 5 mm, and a depth profile resolution of only about a millimeter due to its U-Shape formed magnet. To achieve a flat, sensitive NMR volume at a greater distance removed from the magnet surface with a higher depth resolution a new magnet design was developed.
The new magnet design provided a magnetic assembly for an NMR apparatus, including a plurality of primary permanent magnets disposed in an array about an axis (hereafter “longitudinal axis”), the arrangement and/or characteristics of the plurality of magnets being such so as to create a zone of homogeneous magnetic field at some location along the axis forward of the array (and into the material when provided). The assembly can include a secondary permanent magnet located along the longitudinal axis, at least partly within the array of primary magnets.
As shown in FIG. 1, an exemplary prior art NMR-MOUSE 1005 provides a portable open NMR sensor equipped with a permanent magnet geometry that generates a highly uniform gradient perpendicular to the scanner surface. A frame 1007 with horizontal plane 1006 supports the specimen and remains stationary during the test. A flat sensitive volume of the specimen is excited and detected by a surface RF coil 1012 placed on top of the magnet 1010 at a position that defines the maximum penetration depth into the specimen. By repositioning the sensitive slice across the specimen by means of a high precision lift 1008, the scanner can produce one-dimensional profiles of the specimen's structure with high spatial resolution. If necessary the depth can be adjusted using the spacer 1011.
FIG. 2 shows an exemplary absorbent article specimen 1000 prepared for use with the exemplary prior art NMR-MOUSE 1005. The garment facing side of the specimen 1003 is mounted on a 50 mm×50 mm×0.30 mm glass slide 1001 using a 40.0 mm by 40.0 mm piece of double-sided tape 1002 (tape must be suitable to provide NMR signal amplitude). A top cap 1004 is prepared by adhering two 50 mm×50 mm×0.30 mm glass slides 1001 together using a 40 mm by 40 mm piece of two-sided tape 1002. The cap is then placed on top of the specimen. The two tape layers are used as functional markers to define the dimension of the specimen by the instrument. As can be understood, the prior art absorbent article system does not easily allow for the analysis of absorbent articles that expand. Additionally, a typical RF pulse sequence used in prior art NMR devices, such as the NMR-MOUSE system, is the Carr-Purcell-Meiboom-Gill (CPMG) pulse train. A pulse sequence is a visual representation of the pulses and delays used in a NMR experiment. A pulse is a collection of oscillating waves with a broad range of frequencies used to rotate the bulk magnetization. Most pulse sequences have more than one pulse which can help for signal enhancement and measuring relaxation times by separation of NMR interactions.
One of skill in the art will recognize that the deterioration of an NMR signal is analyzed in terms of two separate processes, each with their own time constants. One process, associated with T1, is responsible for the loss of signal intensity. The other process, associated with T2, is responsible for the broadening of the signal. Two distinguishable relaxation times in NMR are the longitudinal relaxation with a characteristic time T1, which is also known as Spin-Lattice relaxation, and transverse relaxation with a characteristic time T2, which is also known as Spin-Spin relaxation. The longitudinal relaxation is the time needed for magnetization in z direction to build up and reach an equilibrium state again (Meq). The build-up rate of magnetization in z direction is proportional to its deviation from the thermal equilibrium state. Transverse relaxation corresponds to the loss of magnetization in the transverse plan. Stated more formally, T1 is the time constant for the physical processes responsible for the relaxation of the components of the nuclear spin magnetization vector M parallel to the external magnetic field, B0 (which is conventionally oriented along the z axis). T2 relaxation affects the components of M perpendicular to B0. In conventional NMR spectroscopy T1 determines the recycle time, the rate at which an NMR spectrum can be acquired. Values of T1 range from milliseconds to several seconds.
Due to the inhomogeneous static field generated by the open geometry of the profile NMR MOUSE, the free induction decay (FID) (i.e., the observable NMR signal generated by non-equilibrium nuclear spin magnetization precessing about the magnetic field) is too short and not detectable. In order to overcome this problem the CPMG pulse sequence is the most frequently used with single-sided NMR. The CPMG pulse sequence generally consists of a 90° pulse followed by 180° pulses that create a train of spin echoes. This sequence acts to refocus, or regain signal loss due to B0 field inhomogeneity. The initial amplitude of the decay can be related to spin density, while the effective relaxation time T2,eff can be extracted by fitting an exponential function to the signal decay.
An exemplary prior art NMR-MOUSE is the Profile NMR-MOUSE model PM25 with High-Precision Lift available from Magritek Inc., San Diego, Calif. Exemplary requirements for the NMR-MOUSE are a nominal 50-100 μm resolution in the z-direction, a measuring frequency of 13.5 MHz, a maximum measuring depth of 25 mm, a static gradient of 8 T/m, and a sensitive volume (x-y dimension) of 40 mm by 40 mm.
However, several problems exist with the prior art NMR-MOUSE device as would relate to the quantification of two- and three-dimensional distribution, and the kinetics of fluid redistribution associated with absorbent articles and the components thereof. First, it was found that the prior art NMR-MOUSE systems that use current, published CPMG pulse sequences cannot be used for the fast quantification of fluid distribution or fluid movement inside an absorbent article (e.g., diapers and catamenials) as the relaxation times T1 and T2 are strongly dependent upon the characteristics of the absorbent article and the components thereof (such as foams, super absorber, pulp, nonwovens, etc.) and their respective saturation levels. To enable the quantification of liquids in absorbent articles and the components thereof, CPMG pulse sequences that can make the T1 and T2 times independent of the absorbent articles and the components thereof and their relative saturation levels that can enable the fast quantification of liquids inside absorbent articles and components thereof is required.
Second, the current NMR-MOUSE equipment is not suitable for measuring absorbent articles that have, or exhibit, expansive properties. For example, a typical absorbent article such as a diaper, will expand (i.e., swell) when an insult is applied to the surface and the insult migrates inward and is absorbed and/or retained by the constituents forming the absorbent article. This is similar to the observed swelling of a sponge when the surface is insulted by a fluid. The fluid migrates into the sponge and the sponge swells. Swelling causes an absorbent article and the material therein to expand upward (i.e., away from the zone of NMR measurement). Thus, the moisture composition of the portion of the absorbent article disposed inside the static sensitive volume changes. Swelling of an absorbent article due to fluid absorption can have two net effects on NMR measurements regardless of the position chosen. First, the local density is reduced (since the quantity of fluid inside the swelling material is reduced by the same factor as the swelling factor and following swelling kinetics), while at the same time fluid that was already past the sensitive volume can re-enter it from below. This results in potentially confounding factors in the interpretation of kinetic measurements when the absorbent article swelling rate and its effect cannot be determined by a parallel independent NMR test. When measuring the characteristics of swellable objects, such as absorbent articles (diapers, catamenials, etc.) the expansion of the object and ensuing migration of the insulted surface away from the surface of the NMR prevents the observation and analysis of fluid migration into the absorbent article as point of reference (e.g., the insulted surface) having the fluid disposed thereon migrates away from the NMR surface.
Therefore, a need exists and it would be beneficial to provide a new device that can enhance the ability of low-field NMR devices to measure, analyze, and evaluate the migration of fluids into an absorbent article by maintaining the point of reference at a fixed location relative to the NMR. Such a device can improve the ability to map fluid migration through an absorbent article necessary to enhance the development of better quality absorbent articles as well as the materials used to manufacture absorbent articles.